To be honest, this project is a lot of work and i will outline why.
The design of the site will need to be done, and probably seperate from the custom computer creator as this will be only one small part of the site.
The flash for the customer computer builder can be quite basic, with a white background the options and nice border to house it.
you are going to need someone to set this up and then integrate the design into oscommerce, and then integrate the flash to work with the custom computer creator, do you already have the code for this? Or do you need it coding, its a big job to create it in php let alone plug it into flash and have it running from the DB too.
I think your budget of $500 might just get you the design and flash, but I would budget double that at least to get everything you want upto a good standard
Also you dont need to use flash, it means people will have to have the flash plugin to use your store, there are other options which in my opinion would be better for you.
We specialise in Oscommerce work and I can put a price together for you if you want which will include everything from hosting to running the comp here on DO.